The present invention relates in general to material handling vehicles such as walkie/rider pallet trucks commonly used for picking stock in large warehouses and, more particularly, to systems and methods for providing steering assistance on such vehicles.
A walkie/rider pallet truck includes generally, a set of load carrying forks, a power unit having at least one motor to power the truck, a steerable wheel and corresponding steering control mechanism to effect the direction of travel of the truck when in motion, and a platform onto which an operator may step and ride while controlling the truck. The steering control mechanism normally includes a movable steering arm, referred to as a tiller handle, which is mechanically coupled to the steerable wheel. The tiller handle includes the operational controls necessary for operating the truck, such as controls for raising and lowering the forks and rotatable twist grips or comparable devices for controlling the speed and direction (forward or reverse) of the truck.
To drive the truck, the operator steps onto the platform and grasps the handle, moving the steering arm into a truck operating range of positions between a generally vertical (up) braking position and a generally horizontal (down) braking position. The operator then actuates the appropriate controls to select a direction (if necessary) and twists one of the rotatable twist grips to accelerate the truck. Steering is effected by transitioning the tiller handle side to side in a generally horizontal plane. If the operator releases the handle, a deadman brake mechanism forces the arm to the up braking position, which actuates a spring-loaded brake to stop the truck. The operator can also actuate the brake manually by moving the steering arm to either of the up or down braking positions. Thus, the walkie/rider pallet truck may be in either a braking or non-braking mode, depending on the position of the steering arm within specified braking and operating arcs.
Due to the mechanical coupling arrangement of the steering arm to the steerable wheel, the steering force required to turn the truck varies during operation. When the truck is traveling, especially at relatively higher speeds, the steering force required by an operator to turn the truck is relatively low. However, a considerable amount of steering force is required to turn the steerable wheel when the truck is either at a standstill, or traveling at slow speeds. In addition to the speed of the truck, the required steering force depends upon a number of other factors including the type and condition of the floor, the length of the forks, the load on the forks, and the tire type. As an example, given a typical material handling truck with 96 inch (2.44 meter) long forks and a poly drive tire, such as is used in many warehouse picking applications, the torque required to turn the steerable wheel of a stationary truck may vary from about 100 lbf-ft (135 N m) for no load conditions, to over 400 lbf-ft (542 N m) for loads of 8,000 Lbs. (3,629 kg.) when operating on a brushed concrete floor. The torque required to turn the steerable wheel of a stationary truck on smooth concrete can range from about 75 lbf-ft (102 N) for no load conditions up to about 300 lbf-ft (407 N) for loads of 8000 Lb. (3,629 kg).
The above-described exemplary steering torque requirements assume that the truck is not moving. The lowest force required to turn the tiller handle typically occurs when the tiller handle is positioned in a normal operating position, e.g., 2 ft (61 cm) from the centerline of the steerable wheel when the handle is in a horizontal position. However, as the handle is moved into a more vertical position, such as when turning in tight spaces, the radius of the arc defined by the handle with respect to the steerable wheel becomes much smaller, which results in increasing the required turning force. Accordingly, an operator operating a truck for a full day may become tired, which may cause productivity decreases. Thus, there is a need for a power assist for reducing the tiller handle turning force required by an operator of a material handling vehicle.